Intensity in the true definition of the word, related to lifting weights, is the percentage of your one rep max. This is one component I use when evaluating my progress from week to week.
This is it correct here.
Am I at 105% today ( this is realistic it's called a PR personal record)
Am I at 80% and so on
Just because it's based on a 1 rep max don't mean it is not used in other rep schemes
So if my best lift on the deadlift is 600 1 rep that is 100% anything above is above 100% but don't expect much over I am usually at 103% unless I am competing and that can be up to 110%
Now on that same lift but I an doing a 5x5 the you gauge of you best set of 5 and so on with other rep schemes.
Max effort isn't just 1 rep
Being a Powerlifter I know these things well and well enough to no my ranges
Now on the other had if I am doing singles and at 5 reps this is well below 100%
Intensity is also not just the weight but you as a person and how you attack the weight and to learn this watch Powerlifting videos and you'll the difference in intensity of a gym rat and a competitive Powerlifter.